The digital encoding techniques have proven to be well adapted to the transmission or storage of electrical signals, e.g. voice originated signals, in a noisy environment. Some of these techniques involve embedding coding parameters to be used in the subsequent decoding process, within the coded data. Needless to say that any error on the coding parameters is particularly troublesome, as it affects the whole decoding process made to reconstruct the original analog signal. Consequently, methods should be developed to ascertain the highest robustness to noise for the coding parameters. One way for achieving this goal is obtained by duplicating the parameters for storage or transmission purposes. This is bit consuming and should be avoided when bit savings is a must, such as for instance when the encoding is operated over voice signals and the bit rate needs to be minimized. A great deal of efforts have been paid to developing techniques for lowering the coding bit rate through use of sophisticated methods involving companding operations.
One of these techniques, the Block Companded PCM (BCPCM) technique, has proven to be well adapted to the transmission of voice over noisy channels. This technique involves splitting the flow of samples of the original signal into fixed length blocks, deriving from each block of samples a characteristic term "C" (e.g. the largest magnitude sample) and normalizing/quantizing each sample within the considered block with respect to C. The normalized/quantized block of samples together with a digitally encoded representation of C is all that is needed to define the block. This method has been presented by A. Croisier in a lecture given at the 1974 International Seminar on Digital Communications in Zurich, Switzerland, and entitled "Progress in PCM and Delta Modulation : Block Companded Coding of Speech Signal".
Obviously, any error affecting a parameter C, once used to normalize a block of samples, would affect the quality of the synthesized signal represented by the considered block.
The invention provides a method for protecting BCPCM data integrity by protecting the sensitive coding parameter represented by the C term.
Needless to mention that the method applies to different coding schemes and different coding parameters applied to different types of signals, as well. It was successfully applied to BCPCM voice encoding.